There was a situation brought up on a discussion board recently where a person was very upset. A partner of many months had broken an agreement to share ahead of time the intent to pursue others, had sex with a friend of the poster, and informed the poster after the fact. Via text.
Sending a text after the fact is better than not telling at all, but to inform a partner after, rather than before, as per the agreement they shared, was just an attempt to sidestep a potentially uncomfortable conversation. The poster's partner took the coward's way out by waiting until afterwards to bring it up.
Why? Fulfillment in the moment (despite the potential fallout and damage to follow) is more highly prized by some than the health of on-going, established relationships.
Having been at the whole ethically non-monogamous gig for over a decade, I can honestly say it has never damaged me, or ruined my opportunities for sex, to bring my partners up to speed before proceeding. The idea that the sex evaporates because of a pause is pretty deeply rooted in scarcity thinking, and possibly even predatory behavior.
Not everyone structures their relationships the way the poster and their partner do, so the above won't apply. Most of us DO have some form of agreement within our relationships, however, and the following advice applies across the board: If one is in a situation where the agreements made with a partner chafe, renegotiate BEFORE taking actions that violate the agreement. It's the most loving thing one can do, for oneself, one's partner, and the new person being connected with.